Turbine engines such as steam or gas turbines, for example, are utilized as thermal power engines in engineering for the purpose of converting energy which is stored in a flow of gas into a mechanical energy, in particular into a rotational movement. In order to ensure the reliable operation of turbine engines of said kind, efforts are made to monitor the rotor blades of the blade wheel continuously, particularly during the operation and hence during the rotation of a blade wheel which is arranged in the turbine engine. In this case it is very important to exactly maintain the distance of the rotor blade tips, i.e. the radially outermost edges of the rotor blades, from the wall surrounding the rotor blades (radial clearance). A minimum radial clearance must be satisfied for safety reasons, although too great a radial clearance results in unnecessarily low efficiency. In addition to the radial clearance, the axial distance to wall sections is also important, particularly in the case of blade wheels in which the blade rows are covered by a shroud. Because these variables change due to various dynamic influencing factors, continuous monitoring of the radial clearance and the axial clearance is important during operation. The size of the radial clearance can be monitored e.g. by means of capacitive sensors which are positioned close to and almost touching the blade tips. However, these sensors are limited in terms of accuracy, spatial resolution and service life.
DE 197 05 769 A1 discloses a device for monitoring radial and axial clearance in a turbine engine. In this case use is made of a radar system comprising a transmit and receive unit from which electromagnetic waves having a fixed frequency are directed through a waveguide onto a blade wheel of the turbine engine. In this case the waveguide is ducted through the housing which surrounds the blade wheel and is fixed there. The outlet of the waveguide is arranged very closely over the rotor blade tips of the blade wheel, such that it is possible to determine from the reflection of the emitted electromagnetic waves the distance of a rotor blade tip from the waveguide end and hence from the wall surrounding the rotor blade. The determining is done by means of an evaluation of the phases of the reflected electromagnetic waves. The distance is determined by determining the phase difference between emitted and reflected microwaves.
Extreme thermal conditions can prevail in the region of the blade wheel within the housing during operation, particularly in gas turbines. In the case of gas turbines, temperatures of approximately 1200° C. usually occur in the flow channel. These extreme temperatures place particular demands on the nature of the waveguide, which must be embodied such that it exhibits temperature stability at the same time as having a low damping capacity for the electromagnetic waves to be guided. Materials having high temperature stability, e.g. superalloys, are generally characterized by a very high damping capacity for the electromagnetic waves to be guided, while materials having a low damping capacity, e.g. copper, are unstable at extreme temperatures of the specified level.